Fix this bug: playing Arena the field score is never updated in the scoreboard. The...

Fix this bug: playing Arena the field score is never updated in the scoreboard. The cause is a typo, it was used player_cnt (count of CA players) instead of player_count (count of players in every game mode). That's why I've also renamed player_cnt in ca_players.

While I was at it I've made a small clean up to arena.qc, trimming some trailing space and converting some
FOR_EACH_CLIENT(self) if (self.classname == "player")
into
FOR_EACH_PLAYER(self).